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Influenza Types

More H3N2v Cases Reported, Still Linked to Pig Exposure

The information contained in this web update reflects the situation at the time of posting. It may not reflect the current situation. Please see Influenza A (H3N2) Variant Virus Outbreaks for the most recent information related to H3N2v.

CDC Continues to Recommend Precautions When Interacting with Pigs, Especially for High Risk Persons

August 17, 2012 -- Today, 71 additional cases of H3N2v are being reported, bringing the total number of such infections since July 2012 in the United States to 224. The Centers for Disease Control and Prevention (CDC) continues to monitor this situation closely and reports that confirmed cases with this new virus continue to be associated with exposure to pigs.

According Dr. Joseph Bresee, “We expect the number of H3N2v cases to rise since this virus has been found in pigs in a number of U.S. states per the USDA and there is so much interaction between people and pigs in fair settings at this time of year.” But Bresee warns against placing too much emphasis on case counts. “Rather than focusing on case counts, it’s important to look at the kind of spread that’s taking place and the severity of illness that’s occurring. The good news is that the main risk factor for H3N2v virus infection continues to be exposure to pigs. This H3N2v virus is not spreading readily from person-to-person and illness so far has been similar to seasonal flu.” Dr. Bresee is Chief of the Epidemiology and Prevention Branch in CDC’s Influenza Division.

This H3N2v virus – which contains genes from avian, swine and human viruses and has acquired the 2009 H1N1 pandemic virus M gene – was found in U.S. pigs in 2010 and in people in 2011. (See “USDA Swine Influenza Surveillance Update” below for more information on this virus in U.S. swine.)

Acquisition of the M gene may make this H3N2v virus spread more easily to and between humans, compared with other variant influenza viruses, which is one reason CDC is watching this situation carefully. The frequency of H3N2v virus infections has increased substantially since the virus was detected in humans last year. “But,” Bresee reiterates, “a key thing we are looking for is whether the virus is spreading from person-to-person. That kind of change would affect our risk assessment. So far, we are not seeing that and we are looking carefully.” States are investigating influenza-like-illness (ILI) associated with fairs and ILI more broadly in the community in persons without pig exposures, and so far there has been no evidence of community spread. In addition, CDC’s seasonal influenza surveillance systems are not detecting any unusual ILI nationally at this time.

The 71 cases reported this week are from the states of Illinois (2), Indiana (18), Michigan (1), Ohio (41), Pennsylvania (4), West Virginia (3) and Wisconsin (2). These are the first reports of H3N2v with the pandemic M gene from Michigan and Wisconsin and the first such reports for 2012 from Pennsylvania and West Virginia, which reported cases of H3N2v with the pandemic M gene in 2011. All cases reported this week continue to have exposure to pigs prior to getting sick. Cumulative totals for 2011 and 2012 by state are available in the H3N2v case count table.

CDC has specific recommendation this summer for people attending fairs . According to Bresee, among other things, “it’s probably best this fair season for people at high risk of flu complications to stay away from swine barns this summer when they are at the fair.” CDC and USDA also have issued a fact sheet entitled “Issues for Fair Organizers to Consider When Planning Fairs.”

People who are at high risk of serious complications from influenza, including from H3N2v virus infection, include: children younger than 5 years, people 65 years and older, pregnant women, and people with certain chronic medical conditions (like asthma, diabetes, heart disease, weakened immune systems, and neurological or neurodevelopmental conditions).

Signs and symptoms of seasonal influenza illness include fever, cough, sore throat, runny or stuffy nose, muscle or body aches, headaches and fatigue and illness can range from mild to severe. “We expect to see the same range of illness with H3N2v virus infection,” Bresee says. “While most cases are reported to have been mild and people have recovered on their own without needing treatment, some hospitalizations have occurred as well and we should be prepared to see hospitalizations and even possibly some deaths, as we do for seasonal flu,” Bresee cautions. CDC also has issued information for clinicians on H3N2v, which underscores the importance of rapid antiviral treatment of H3N2v virus infections in high risk patients.

Though rare, influenza viruses can spread from people to pigs and pigs to people, however, influenza viruses have not been shown to be transmissible to people through eating pork (pig meat).

The USDA, in cooperation with State and industry, conducts voluntary surveillance for SIV in the US. When viruses like the H3N2v viruses with the 2009 H1N1 pandemic M gene recently isolated in humans are detected in swine, they are called “H3N2pM.” The agency first identified H3N2pM virus isolates collected in late 2010 and have continued to find it across the U.S. since then.

USDA’s SIV Surveillance Program has tested 12,662 samples from 3,766 swine diagnostic lab submissions collected from October 1, 2010 through July 31, 2012. Over that time period, 1,488 case submissions have been identified as positive for Influenza A infection. Overall, 73 H3N2 positive submissions were detected in FY2011 (October 1, 2010 to September 30, 2011) and 138 in FY2012 from October 1, 2011 to July 31, 2012. 57 Of the 138 H3N2 cases identified in FY2012 and tested to date contain the pandemic M gene and were classified as H3N2pM. Several additional H3N2-positive submissions remain under advanced diagnostic processes. USDA reports that additional detailed characterization results will be reported as results are compiled.